Recently, with the growth of urban and industrial areas, and the shortage of land and high quality materials, demand for use of marginal lands is increasing. Additionally, techniques and equipment for improvement of loose and soft soil have been proposed. Generally, soft cohesive soils have two main characteristics, the first one is the low shear strength, and the second one is large settlement. Furthermore, loose granular soils have the great potential of liquefaction.
In the last few decades, compaction using sandy or gravelly columns, pile, piers, etc., has been conducted all over the world as a technical and an economical method. In practice, various methods of compacting gravelly piers were founded on one of the construction replacement, construction displacement, or combination methods. However, the final products, construction process, the configuration, and the effect on the relative density of gravel and matrix soil, which were constructed by each of the aforementioned methods, are very different. Generally, the gravel piers constructed by construction methods which are based on the type of loading, mechanical and physical characteristics of soil layers, and environmental conditions, are divided into multiple categories, for example, stone columns, compaction piles, rammed aggregate piers, etc.
Typically, stone columns are constructed by replacement of loose material with gravelly material by two methods of vibrating replacement and vibrating compaction. Compaction piles are constructed based on displacement mechanism by means of excavating a hole in the ground and making a radial compaction for the surrounding soil and filling it by sandy or gravelly material by two methods of sand compaction piles and gravel compaction piles. Rammed aggregate piers are made of methods based on the combination of replacement and displacement, and by means of excavating a hole by mechanical auger, filling it by gravelly material and making a radial compaction in the layers.
Generally, aggregate piers with 0.6-1.5 inch diameter and 2-10 meters length are compacted in a square or triangular pattern in a weak soil base. Aggregate piers with depths of more than 10 meters are not as economical as deep concrete foundations. However, there are many reports signifying the construction of aggregate piers with 10-30 meters length. Compaction piers compact the soil by two mechanisms, for example, volume displacement of the soil equal to the volume of the pier, and the soil compaction around the pier due to vibrations caused by driving the pier.
Conventional construction methods of aggregate piers that are used in most companies include vibro-replacement method, Fanki method of rammed aggregate piers, Fox and Lowton method of rammed aggregate piers, etc. In the vibro-replacement wet method, a hole is formed in the ground by jetting a probe down to the desired depth. The uncased hole is flushed out and then stone is added in 0.3-1.2 m increments and made dense by means of an electrically or hydraulically actuated vibrator located near the bottom of the probe. The wet process is generally used where borehole stability is questionable. Therefore, it is suited for sites underlain by very soft to firm soils and a high ground water table.
The wet process produces a great deal of environmental pollution especially in limited area of urban lands due to exit slurry from the wells and the need for surrounding areas for construction of sediment basins. Furthermore, the wet process cannot be applied to loose soils and soils with low bearing strength. In such soils, the probability of destruction of buildings surrounding the excavated area is higher. In areas having water shortages, it is disadvantageous to use water that is mandatory for the wet process.
Therefore, a method of construction of aggregate piers, which is environment friendly, applicable to loose soils, and applicable in regions having water shortages, is required. Moreover, heavy machinery and equipment are needed for the vibro-replacement wet method. The great height and volume of heavy machinery and inability of applying them in urban areas due to space constraints renders the process disadvantageous. An aggregate compacting system, which occupies minimum space and is easy to handle, is required.
In the vibro displacement dry method, the jetting water during initial formation of the hole is absent. For using the vibro-displacement dry method, the vibrated hole must be able to stand open upon extraction of the probe. Therefore, for vibro-displacement to be possible, soils must exhibit shear strengths in excess of about 40-60 kPa, with a relatively low ground water table being present at the site. Stabilization is made possible by using a “bottom feed” type vibrator. Eccentric tubes adjacent to the probe allow delivery of stone, sand or concrete to the bottom of the excavated hole without extracting the vibrator. Using this method, the vibrator serves as a casing, which prevents collapse of the hole. This method cannot be used in areas having a high water table. A system and method of compacting aggregate piers, which are deployable in regions having high water table, is required.
In the Fanki method, rammed aggregate piers are constructed by either driving an open or closed end pipe in the ground or boring a hole. A mixture of sand and stone is placed in the hole in increments. The mixture is rammed in using a heavy, falling weight. Disturbance and subsequent remolding of sensitive soils by the ramming operation, however, may limit its utility in these soils. Additional infrastructure is required when using heavy machinery devices in unstable loose soils. The great height and volume of instruments and machinery devices render them unusable in urban areas due to constraints of passages. Inability to apply them in urban areas due to high vibrations because of compaction strikes and the probability of great destructions is another disadvantage. In limited urban areas, which urge rehabilitation of layers with little thickness, the mentioned method has concerns and is not economical.
The Fox and Lowton method of rammed aggregate piers (RAPs) are one of the soft soils reinforcement techniques used to reduce intolerable settlements. Additionally, the method serves to improve the bearing capacity and stiffness in various building projects. The construction process of rammed aggregate piers consists of cavity drilling, making end-resistant bulbs, and implementing pier shafts. End-resistant bulbs and pier shafts are constructed using layers of open graded and well-graded gravel, respectively. The nominal thickness of aggregate layers is about 0.3 m and each layer is compacted using a specially designed, beveled tamper connected to a hydraulic hammer. The hydraulic hammer delivers between 1-2 million ft-lbs of energy to the RAP at approximately 400 blows per minute. Because of aggregate compaction, the soft soil at the end bulb is to deform downward and laterally, and in the next aggregate layers, the soft soil around the pier deform laterally under compression. In this method, excavation of the well by mechanical auger for each pier is mandatory. Moreover, the wells excavated in loose and collapsible soils with high water level are highly unstable. The casing pipe must be applied individually for each pier followed by removing the soil. This affects the speed of the method and results in delays in project execution. Additionally, this method suffers from lack of technical and economic feasibility in some cases. Thus, a method which is applicable in loose soils with high water level, allows quick implementation, and is technically and economically feasible, is required.
Hence, there is a long felt but unresolved need for a method of construction of aggregate piers, which is environment friendly, applicable to loose soils, and applicable in regions having water shortages. Furthermore, there is a need for an aggregate compacting system, which occupies minimum space and is easy to handle. Moreover, there is a need for a system and method of compacting aggregate piers, which are deployable in regions having high water table. Furthermore, there is a need for a method, which is applicable in loose soils with high water level, allows quick implementation, and is technically and economically feasible.